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Hi, my name is Mike and I’m an support engineer with Agilent.
This short video will describe how the 34410 and the 34411 makes capacitance measurements.
This is not a typical measurement made by DMM’s and it’s only recently that Agilent has
developed some DMM’s that will make and measure capacitors. The technique we use is
to apply a known current to the capacitor as shown here in the charging phase. We charge
up the capacitor to a known voltage. It’s during this phase that we actually make the
measurement. However we do need to discharge the capacitor in preparation for making another
capacitance measurement. So the total cycle involves a charge phase and a discharge phase.
Let’s take a look at the charging phase where we actually make the measurement.
This is a standard equation describing a charging capacitor and we know all the values associated
with that. We just need to make some measurements along this charging curve to determine the
capacitance and we do that twice where we will make a DV/Dt measurement. So here’s
one short aperture where we’re making the measurement and we actually make a measurement
at the beginning and at the end of the short aperture to understand what the change in
voltage and the change in time is. And we do that similarly at another place on the
charging graph. Again, we’re looking at the difference in voltage over the small aperture
time or time change. With those four points, we can accurately calculate the capacitance.
Here we show part of the specification sheet for the 34410 and 11 for capacitance.
This is very similar to the standard voltage measurements. We have multiple ranges that we can
measure capacitors. We know that we’ll have different test currents that we apply as we’re charging
the capacitor. We have 24-hour, 90-day, and one-year specifications with a percent of
reading and a percent of range error term as well as an additional temperature coefficient
term if that is needed.
The value, if you measure capacitance with a DMM may be different than what you get with
an LCR meter and that’s because the techniques are slightly different. As you see here, we
use a charging technique and calculate for a typical curve. With an LCR meter, you actually
apply different AC frequencies between typically 100 hertz and 100 kilohertz to make the measurements.
In most cases, neither of these are completely accurate because rarely is a capacitor tested
at the frequency that you’re using it in the application but they both give good
approximations to the capacitance.
This concludes the short video describing how the 34410 and 11 make capacitance measurements.